Additional Inherited Members

Detailed Description

The QGLWidget class is a widget for rendering OpenGL graphics.

QGLWidget provides functionality for displaying OpenGL graphics integrated into a Qt application. It is very simple to use. You inherit from it and use the subclass like any other QWidget, except that you have the choice between using QPainter and standard OpenGL rendering commands.

QGLWidget provides three convenient virtual functions that you can reimplement in your subclass to perform the typical OpenGL tasks:

paintGL() - Renders the OpenGL scene. Gets called whenever the widget needs to be updated.

resizeGL() - Sets up the OpenGL viewport, projection, etc. Gets called whenever the widget has been resized (and also when it is shown for the first time because all newly created widgets get a resize event automatically).

initializeGL() - Sets up the OpenGL rendering context, defines display lists, etc. Gets called once before the first time resizeGL() or paintGL() is called.

If you need to trigger a repaint from places other than paintGL() (a typical example is when using timers to animate scenes), you should call the widget's updateGL() function.

Your widget's OpenGL rendering context is made current when paintGL(), resizeGL(), or initializeGL() is called. If you need to call the standard OpenGL API functions from other places (e.g. in your widget's constructor or in your own paint functions), you must call makeCurrent() first.

QGLWidget provides functions for requesting a new display format and you can also create widgets with customized rendering contexts.

You can also share OpenGL display lists between QGLWidget objects (see the documentation of the QGLWidget constructors for details).

Note that under Windows, the QGLContext belonging to a QGLWidget has to be recreated when the QGLWidget is reparented. This is necessary due to limitations on the Windows platform. This will most likely cause problems for users that have subclassed and installed their own QGLContext on a QGLWidget. It is possible to work around this issue by putting the QGLWidget inside a dummy widget and then reparenting the dummy widget, instead of the QGLWidget. This will side-step the issue altogether, and is what we recommend for users that need this kind of functionality.

On Mac OS X, when Qt is built with Cocoa support, a QGLWidget can't have any sibling widgets placed ontop of itself. This is due to limitations in the Cocoa API and is not supported by Apple.

Overlays

The QGLWidget creates a GL overlay context in addition to the normal context if overlays are supported by the underlying system.

If you want to use overlays, you specify it in the format. (Note: Overlay must be requested in the format passed to the QGLWidget constructor.) Your GL widget should also implement some or all of these virtual methods:

These methods work in the same way as the normal paintGL() etc. functions, except that they will be called when the overlay context is made current. You can explicitly make the overlay context current by using makeOverlayCurrent(), and you can access the overlay context directly (e.g. to ask for its transparent color) by calling overlayContext().

On X servers in which the default visual is in an overlay plane, non-GL Qt windows can also be used for overlays.

Painting Techniques

As described above, subclass QGLWidget to render pure 3D content in the following way:

Overpainting 2D content on top of 3D content takes a little more effort. One approach to doing this is shown in the Overpainting example.

Threading

As of Qt version 4.8, support for doing threaded GL rendering has been improved. There are three scenarios that we currently support:

1. Buffer swapping in a thread.

Swapping buffers in a double buffered context may be a synchronous, locking call that may be a costly operation in some GL implementations. Especially so on embedded devices. It's not optimal to have the CPU idling while the GPU is doing a buffer swap. In those cases it is possible to do the rendering in the main thread and do the actual buffer swap in a separate thread. This can be done with the following steps:

1. Call doneCurrent() in the main thread when the rendering is finished.

4. Call doneCurrent() in the swapping thread and notify the main thread that swapping is done.

Doing this will free up the main thread so that it can continue with, for example, handling UI events or network requests. Even if there is a context swap involved, it may be preferable compared to having the main thread wait while the GPU finishes the swap operation. Note that this is highly implementation dependent.

2. Texture uploading in a thread.

Doing texture uploads in a thread may be very useful for applications handling large amounts of images that needs to be displayed, like for instance a photo gallery application. This is supported in Qt through the existing bindTexture() API. A simple way of doing this is to create two sharing QGLWidgets. One is made current in the main GUI thread, while the other is made current in the texture upload thread. The widget in the uploading thread is never shown, it is only used for sharing textures with the main thread. For each texture that is bound via bindTexture(), notify the main thread so that it can start using the texture.

In Qt 4.8, it is possible to draw into a QGLWidget using a QPainter in a separate thread. Note that this is also possible for QGLPixelBuffers and QGLFramebufferObjects. Since this is only supported in the GL 2 paint engine, OpenGL 2.0 or OpenGL ES 2.0 is required.

QGLWidgets can only be created in the main GUI thread. This means a call to doneCurrent() is necessary to release the GL context from the main thread, before the widget can be drawn into by another thread. Also, the main GUI thread will dispatch resize and paint events to a QGLWidget when the widget is resized, or parts of it becomes exposed or needs redrawing. It is therefore necessary to handle those events because the default implementations inside QGLWidget will try to make the QGLWidget's context current, which again will interfere with any threads rendering into the widget. Reimplement QGLWidget::paintEvent() and QGLWidget::resizeEvent() to notify the rendering thread that a resize or update is necessary, and be careful not to call the base class implementation. If you are rendering an animation, it might not be necessary to handle the paint event at all since the rendering thread is doing regular updates. Then it would be enough to reimplement QGLWidget::paintEvent() to do nothing.

As a general rule when doing threaded rendering: be aware that binding and releasing contexts in different threads have to be synchronized by the user. A GL rendering context can only be current in one thread at any time. If you try to open a QPainter on a QGLWidget and the widget's rendering context is current in another thread, it will fail.

Note that under X11 it is necessary to set the Qt::AA_X11InitThreads application attribute to make the X11 library and GLX calls thread safe, otherwise the above scenarios will fail.

In addition to this, rendering using raw GL calls in a separate thread is supported.

OpenGL is a trademark of Silicon Graphics, Inc. in the United States and other countries.

The parent and widget flag, f, arguments are passed to the QWidget constructor.

If shareWidget is a valid QGLWidget, this widget will share OpenGL display lists and texture objects with shareWidget. But if shareWidget and this widget have different formats, sharing might not be possible. You can check whether sharing is in effect by calling isSharing().

The initialization of OpenGL rendering state, etc. should be done by overriding the initializeGL() function, rather than in the constructor of your QGLWidget subclass.

The parent and widget flag, f, arguments are passed to the QWidget constructor.

If shareWidget is a valid QGLWidget, this widget will share OpenGL display lists and texture objects with shareWidget. But if shareWidget and this widget have different formats, sharing might not be possible. You can check whether sharing is in effect by calling isSharing().

The initialization of OpenGL rendering state, etc. should be done by overriding the initializeGL() function, rather than in the constructor of your QGLWidget subclass.

The format argument specifies the desired rendering options. If the underlying OpenGL/Window system cannot satisfy all the features requested in format, the nearest subset of features will be used. After creation, the format() method will return the actual format obtained.

The parent and widget flag, f, arguments are passed to the QWidget constructor.

If shareWidget is a valid QGLWidget, this widget will share OpenGL display lists and texture objects with shareWidget. But if shareWidget and this widget have different formats, sharing might not be possible. You can check whether sharing is in effect by calling isSharing().

The initialization of OpenGL rendering state, etc. should be done by overriding the initializeGL() function, rather than in the constructor of your QGLWidget subclass.

Converts the image img into the unnamed format expected by OpenGL functions such as glTexImage2D(). The returned image is not usable as a QImage, but QImage::width(), QImage::height() and QImage::bits() may be used with OpenGL. The GL format used is GL_RGBA.

void QGLWidget::glInit () [virtual protected]

Returns an image of the frame buffer. If withAlpha is true the alpha channel is included.

Depending on your hardware, you can explicitly select which color buffer to grab with a glReadBuffer() call before calling this function.

void QGLWidget::initializeGL () [virtual protected]

This virtual function is called once before the first call to paintGL() or resizeGL(), and then once whenever the widget has been assigned a new QGLContext. Reimplement it in a subclass.

This function should set up any required OpenGL context rendering flags, defining display lists, etc.

There is no need to call makeCurrent() because this has already been done when this function is called.

void QGLWidget::initializeOverlayGL () [virtual protected]

This virtual function is used in the same manner as initializeGL() except that it operates on the widget's overlay context instead of the widget's main context. This means that initializeOverlayGL() is called once before the first call to paintOverlayGL() or resizeOverlayGL(). Reimplement it in a subclass.

This function should set up any required OpenGL context rendering flags, defining display lists, etc. for the overlay context.

There is no need to call makeOverlayCurrent() because this has already been done when this function is called.

bool QGLWidget::isSharing () const

Returns true if this widget's GL context is shared with another GL context, otherwise false is returned. Context sharing might not be possible if the widgets use different formats.

Handles paint events passed in the event parameter. Will cause the virtual paintGL() function to be called.

The widget's rendering context will become the current context and initializeGL() will be called if it hasn't already been called.

void QGLWidget::paintGL () [virtual protected]

This virtual function is called whenever the widget needs to be painted. Reimplement it in a subclass.

There is no need to call makeCurrent() because this has already been done when this function is called.

void QGLWidget::paintOverlayGL () [virtual protected]

This virtual function is used in the same manner as paintGL() except that it operates on the widget's overlay context instead of the widget's main context. This means that paintOverlayGL() is called whenever the widget's overlay needs to be painted. Reimplement it in a subclass.

There is no need to call makeOverlayCurrent() because this has already been done when this function is called.

You can use this method on both visible and invisible QGLWidget objects.

This method will create a pixmap and a temporary QGLContext to render on the pixmap. It will then call initializeGL(), resizeGL(), and paintGL() on this context. Finally, the widget's original GL context is restored.

The size of the pixmap will be w pixels wide and h pixels high unless one of these parameters is 0 (the default), in which case the pixmap will have the same size as the widget.

If useContext is true, this method will try to be more efficient by using the existing GL context to render the pixmap. The default is false. Only use true if you understand the risks. Note that under Windows a temporary context has to be created and usage of the useContext parameter is not supported.

Overlays are not rendered onto the pixmap.

If the GL rendering context and the desktop have different bit depths, the result will most likely look surprising.

Note that the creation of display lists, modifications of the view frustum etc. should be done from within initializeGL(). If this is not done, the temporary QGLContext will not be initialized properly, and the rendered pixmap may be incomplete/corrupted.

x and y are specified in window coordinates, with the origin in the upper left-hand corner of the window. If font is not specified, the currently set application font will be used to render the string. To change the color of the rendered text you can use the glColor() call (or the qglColor() convenience function), just before the renderText() call.

The listBase parameter is obsolete and will be removed in a future version of Qt.

Note: This function clears the stencil buffer.

Note: This function is not supported on OpenGL/ES systems.

Note: This function temporarily disables depth-testing when the text is drawn.

x, y and z are specified in scene or object coordinates relative to the currently set projection and model matrices. This can be useful if you want to annotate models with text labels and have the labels move with the model as it is rotated etc.

Note: This function is not supported on OpenGL/ES systems.

Note: If depth testing is enabled before this function is called, then the drawn text will be depth-tested against the models that have already been drawn in the scene. Use glDisable(GL_DEPTH_TEST) before calling this function to annotate the models without depth-testing the text.

This virtual function is used in the same manner as paintGL() except that it operates on the widget's overlay context instead of the widget's main context. This means that resizeOverlayGL() is called whenever the widget has been resized. The new size is passed in width and height. Reimplement it in a subclass.

There is no need to call makeOverlayCurrent() because this has already been done when this function is called.

void QGLWidget::setAutoBufferSwap ( boolon ) [protected]

If on is true automatic GL buffer swapping is switched on; otherwise it is switched off.

If on is true and the widget is using a double-buffered format, the background and foreground GL buffers will automatically be swapped after each paintGL() call.

void QGLWidget::updateGL () [virtual slot]

void QGLWidget::updateOverlayGL () [virtual slot]

Updates the widget's overlay (if any). Will cause the virtual function paintOverlayGL() to be executed.

The widget's rendering context will become the current context and initializeGL() will be called if it hasn't already been called.

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